1. Field of the Invention
This invention relates to hydroelectric power-generating devices that use underwater currents to drive electricity-generating turbines.
2. Description of the Prior Art
The use of underwater power generators for generating electricity from water current flow, such as rivers and oceans, is known in the art. There are two types of prior devices: stationary turbines and tethered turbines. Stationary types of turbines are comprised of stationary towers based on the ocean floor. Electricity generating turbines are mounted on the towers at a fixed depth, with turbine rotor blades facing the flow of an ocean current. The disadvantages of this type of design are the underwater construction costs and the engineering challenges of deploying towers in deeper waters. Examples of these devices are being tested in Europe as tidal turbines and a few are being tested now in North American waters as of 2007.
Tethered devices are designed to operate underwater, and are kept in place by a tether system that is anchored to the ocean floor. In some cases, a wing (hydrofoil) provides lift and/or ballast tanks provide buoyancy to keep the devices from descending. Some devices use a buoyancy chamber to regulate their overall buoyancy thereby adjusting their operating depths in a current stream. Other devices add a moveable wing that serves as a stabilizer to control the depth of the device. The wing is adjusted to cause the device to dive or surface in response to emergency conditions such as floating debris on a river.
An example of this type of turbine device is an underwater power plant called the Coriolis Project. The design called for a ducted, catenary turbine, 171 meters in diameter, capable of producing 83 MW of electricity. The turbine was to be tethered at a fixed depth.
U.S. Pat. No. 6,091,161 discloses a method of controlling a tethered, underwater, water current-driven turbine, power-generating device. A predetermined maximum depth and a predetermined minimum depth are set. In response to sensing depth of the device, an ascend protocol or a descend protocol are selectively invoked. These protocols maintain an operating depth of the device that is midway between the predetermined maximum depth and the predetermined minimum depth. The turbine includes variable-pitch rotor blades. A maximum allowable drag force load on the turbine rotors is selected. The pitch of the variable-pitch rotor blades on the turbine is adjusted such that the drag force loading of the device does not exceed a maximum design level.
The Manta Raytm is a tethered marine turbine similar to an underwater kite-surf. An asymmetric nozzle provides lift and funnels water to a small turbine near the outlet for generating electricity. The device is described in International Application WO 2006/117830, entitled “Water Turbine In Tethered Asymmetric Nozzle” published Sep. 11, 2006, inventor Francis Allen Farrelly.
The application describes an energy generating device for harnessing submarine currents composed of an asymmetric nozzle capable of directing the water flow towards one or more turbines placed before the said nozzle's outlet. The nozzle's inlet is asymmetric in shape; one surface of the inlet extends beyond its opposite surface, thus acting as a lifting surface producing a force with a component perpendicular to the current flow. The device is moored with a tether-line system, which enables the device to remain in equilibrium, by counteracting the vertical component of the tether's tension, with the vertical lift due to the water flowing through the asymmetric nozzle.
Another underwater turbine device is described in International Application WO 2000/077393, entitled “Dual Hydroturbine Unit” published Dec. 12, 2000, inventor Philippe Vauthier.
The Vauthier application describes an environmentally non-intrusive multiple turbine unit for adjustable deployment in water. The turbines are captured in a shroud having an integrally formed augmentor ring and ballasting member to improve efficiency. The multiple turbine arrangement effectively counteracts the rotational counter-torque of individual turbines, thereby enabling stable deployment of the unit without requiring rigid stabilizing structures.
What is needed is a way of controlling a tethered, underwater, water current-driven turbine, power-generating device in a simplified and cost-effective manner such that the inconsistent and varying ocean currents do not greatly affect the stability, safety and power generating performance. In order to keep installation and maintenance costs of such a device down, it is desirable to provide minimal underwater structures and construction. This means that one must be able to safely bring the device to the surface for maintenance or for replacement of single or multiple turbines without altering the arrays, consisting of a plurality of turbine modules.
It is also desirable to provide for emergencies that require a complete shutdown of the device without immediate surfacing of the device for maintenance or repair.
Submerging further from an operating depth due to inclement weather or other force major events is desired.